Asked whether he would halt sanctions to convince Iran to return to the bargaining table, Biden offered a clear reply in a CBS interview aired Sunday: "No."

The journalist then asked if the Iranians would first have to stop enriching uranium, which drew an affirmative nod from Biden.

The 2015 landmark deal has been hanging by a thread since Donald Trump's decision to withdraw from it in 2018 and reimpose sanctions on Tehran.

Tehran a year later suspended its compliance with most key nuclear commitments to the deal.

The Biden administration has expressed willingness to return to the deal, but insisted that Tehran first resume full compliance.

On January 4, Iran announced it has stepped up its uranium enrichment process to 20 percent purity, far above the 3.67 percent level permitted by the deal, but far below the amount required for an atomic bomb.